STABILITY ASSESSMMENT OF THE UNDERGROUND SETTLING BASIN CAVERNS OF SUPER MADI HYDROELECTRIC PROJECT,NEPAL

Abstract

Super Madi Hydrolectric Project (SMHEP) is located in Kaski District of Nepal. It is a run of river scheme with a installed capacity of 44 MW, net head of 295m and design discharge of 18 m^3⁄s.It has planned to build for the fulfillment to minimize the load-shedding problem of Nepal in the current scenario. This project lies in the lower part of the Higher Himalaya, mainly dominated by high-grade metamorphic rocks like gneiss and schists.Major task involve in this thesis work is to check for proper alignment of existing layout, assessment of stability condition with proper support system. Selection of best alternative alignment of cavern with its best shape and size are also another major work in this thesis. Optimum support estimation for the best alternative has also been done. Conclusion and final recommendations are based on stability condition and degree of rock support requirements.Geological and topographical site condition of headworks restrict for exposed settling basin therefore underground settling basin cavern in the left hill side has been selected. Rock mass in the settling basin area is slightly deformed, foliated micaous and banded gneiss with thin layer of schist. Analysis is based on assumption of ``No significant faults and shear zones across the alignment of settling basin cavern``.Both alternatives with axis orientation of N145E have been selected for the analysis. Shape of the caverns in both alternatives are inverted D. Existing alternative consists of two parallel settling basin caverns with a clear spacing of 9.5 m. Average width and height for both the caverns of existing alternative (Alternative I) are 8.4 m and 15 m whereas for proposed alternative (Alternative II) are 18.3 m and 20 m are respectively.Stress-strength factor plays a vital role for overall stability condition of the cavern. Stress induced problems such as rock bursting and spalling in hard rock whereas squeezing in weak rock is assumed. Some Empirical, Analytical, and Numerical approaches have been used for stability assessment and for designing of proper rock support system.RMR and Q-system of rock mass classification are used to classify the rock masses. Grimstad and Barton (1993) method is used in the analysis of rock bursting problem and squeezing problem. As a Semi-analytical approach ``Hoek and Marionos approach`` has been used for squeezing analysis.Numerical approaches have many benefits over empirical and analytical approaches, specifically in complex geometry like settling basin cavern. Rocscience software for numerical analysis such as Phase^2 and Un-wedge has been used. Generalized Hoek and Brown failure criterion are used to determine the state of stresses, strength factors, and deformations around the periphery of the caverns in Phase^2 . To analyze the wedge failure due to low shear strength of joints, empirical approach suggested by Barton and Bandis is used in the numerical analysis through rocscience software-Unwedge.Comparative study of empirical, analytical, and numerical approaches of analysis have been carried out for assessment of stability conditions. Finally, some recommendations to improve the analysis results have been performed.